In manufacturing a semiconductor device, a display, and the like, lithography is used for exposing and transferring, on a face of a photo resist coated on a material (layer) provided on a substrate made of a semiconductor, glass, resin, or the like, a pattern of an electronic circuit (hereinafter called “pattern”) formed on a photo mask. The art of photolithography is also used in a manufacturing process of a liquid crystal display, which is characterized as follows.
For example, in the photolithography in the manufacturing process of the liquid display, a magnification of exposure is equalized in order to expose a face of a large area at great speed.
Also, there is an advantage that a deep depth of focus can be obtained by using a shorter wavelength than an i-ray (365 nm in wavelength) currently in great use. For obtaining such a short wavelength, an idea of using an eximer laser, for example, can be conceived. However, because an eximer laser is expensive and unstable in oscillation, maintenance thereof is difficult. Therefore an i-ray of an ultra-high-pressure mercury lamp is used, because it is relatively inexpensive, stable in motion, and easily maintainable.
Further, because of a demand for miniaturization of a pattern to be exposed, it is sought to miniaturize a dimension to be exposed while maintaining the magnification and wavelength of exposure. However, so long as an actual chrome mask is used, miniaturization of a dimension to be exposed results in reduction in depth of focus. It is, therefore, attempted to adopt an attenuated type phase shift mask capable of obtaining a relatively deep depth of focus.
The attenuated type phase shift mask comprises a reference area allowing a light radiated from a light source to pass and an amplitude and phase modulation area allowing a part of the light to pass. According to the attenuated type phase shift mask, the light having passed the reference area and the light having passed the phase shift modulation area have inverted phases (with a phase difference of 180°), thereby causing in a boundary of both passing lights a lowering in light intensity due to the phase inversion and a constraining spread toward the bottom of the light intensity distribution in the face to be exposed (“On Art of Photo Mask” issued by Kabushiki Kaisha Kogyo Chosakai on Oct. 20, 1997, pp. 229-232). Therefore, a deeper depth of focus than that of the chrome mask can be obtained.
It is possible to judge the depth of focus of a photo mask generally by seeing the light intensity distribution (aerial image) in the vicinity of an image plane. That is to say, when a change of the light intensity distribution is small with respect to the direction of an optical axis, the depth of focus can be judged to be deep. The depth of focus in the attenuated type phase shift mask can also be evaluated by the light intensity distribution in the vicinity of the image plane, and it is known that a change of the light intensity is smaller than that of the chrome mask with respect to the direction of the optical axis, namely, that the depth of focus is deep. It is known that, when the pattern of the mask is sufficiently large in comparison with the wavelength, the light intensity distribution in the vicinity of the image plane is symmetric (mirror symmetry) with respect to the image plane.
However, the inventor's study revealed that even by the attenuated type phase shift mask, when the dimension of the pattern formed thereon is as large as or smaller than the wavelength of the source light, a change of its light intensity distribution with respect to the optical axis becomes large and the light intensity distribution becomes asymmetric with respect to the image plane. This resulted in the problem of a shallow depth of focus.